DAG-JSON: decodes large negative integers as floats

Current DAG-JSON cannot decode large negative integers as floats. If they are outside the i64 range, they get automatically converted into a float. This means that decoding a number like -11959030306112471732 becomes `Float(-1.1959030306112471e19)` instead of the expected `Integer(-11959030306112471732)`.

Here's an example test case:

```rust
#[test]
fn decode_large_negative_integer() {
let integer: i128 = -11959030306112471732;
let decoded: Ipld = DagJsonCodec.decode(integer.to_string().as_bytes()).unwrap();
assert_eq!(decoded, Ipld::Integer(integer));
}
```

One way is to indeed decode it as integer, another way (which is likely easier) is to have an error if it's outside the i64 range. I think such an error would be better then implicit conversion.

Though I should add that if someone spends time on the current DAG-JSON implementation, that it might be better spend on a new DAG-JSON implementation similar to https://github.com/ipld/serde_ipld_dagcbor.

This means that I'm not sure if this issue ever gets fixed in the current implementation. I just wanted to make sure this issue is tracked somewhere in case someone encounters it.